The present invention relates to steam turbines and, more particularly, to a method and apparatus for reducing the amount of steam flow required by the steam seal system in order to properly “self seal” a double flow combined cycle steam turbine.
Currently available combined cycle systems of the assignee of this invention include single and multi-shaft configurations. Single shaft configurations may include one gas turbine, one steam turbine, one generator and one heat recovery steam generator (HRSG). The gas turbine and steam turbine are coupled to the single generator in a tandem arrangement on a single shaft. Multi-shaft systems, on the other hand, may have one or more gas turbine-generators and HRSG's that supply steam through a common steam header to a single steam turbine generator. In either case, steam is generated in one or more HRSG's for delivery to the condensing steam turbine.
It is well known that when a steam turbine is operating at a load below its self-sealing point, steam from an external supply (i.e., make-up steam) must be provided to the seal steam header to maintain the turbine seals until a self-sealing point is reached.
When a steam turbine “self seals”, it refers to the ability of the turbine to pressurize (i.e., create a vacuum) and “seal” the ends of the double flow low pressure (LP) rotor. When a turbine fails to self seal, it cannot pressurize and create a vacuum at the ends of the LP rotor using its allocated steam. In this instance, additional “makeup” steam is required to feed the steam seal header. The steam flow requirement for the steam seal system, which is supplied by the high pressure (HP) and intermediate pressure (IP) sections of the turbine, is based on the steam flow demand required by the low pressure (LP) turbine section. Hence, if the LP steam flow demand is lowered, then the supply steam from the HP and IP sections can be reduced.
The “makeup” steam taken from the HP and IP sections to feed the steam seal system bypasses the steam path all together, eliminating all possibilities to extract the energy of the steam through the turbine buckets and nozzles. The wasted opportunity costs of this bypassed steam limits the ability of the turbine to reach entitlement (maximum efficiency).
Furthermore, if a turbine experiences a “rub” event, in which the teeth of the metal packing ring make contact with the rotor and become damaged, the radial clearance, or the distance between the teeth and the rotor, increases. This increase in radial clearance causes the required flow, Q, to self seal to increase. If, in fact, the LP packing rings experience a significant rub, then the required flow, Q, to self seal can increase beyond the capability of the HP and IP turbines to supply enough steam to feed a steam seal header (SSH) to seal the newly rubbed LP packing rings.
Therefore, a solution is needed to reduce the source steam flow requirement coming from the HP and IP turbines to feed the steam seal header (SSH) and reduce self-sealing failure probability due to a rub event.